New constraints on the chemical evolution of the dwarf spheroidal galaxy Leo I from VLT spectroscopy^*

¹ Osservatorio Astronomico di Padova, INAF, vicolo dell'Osservatorio 5, 35122 Padova, Italy e-mail: [marco.gullieuszik;enrico.held]@oapd.inaf.it
² European Southern Observatory, Casilla 19001, Santiago 19, Chile e-mail: isaviane@eso.org
³ Joint Astronomy Centre, 660 N. A'ohoku Place, University Park, Hilo, HI 96720, USA e-mail: l.rizzi@jach.hawaii.edu

Received: 23 December 2008
Accepted: 27 March 2009

Abstract

We present the spectroscopy of red giant stars in the dwarf spheroidal galaxy Leo I, aimed at further constraining its chemical enrichment history. Intermediate-resolution spectroscopy in the  triplet spectral region was obtained for 54 stars in Leo I using FORS2 at the ESO Very Large Telescope. The equivalent widths of  triplet lines were used to derive the metallicities of the target stars on the [Fe/H] scale of Carretta & Gratton, as well as on a scale tied to the global metal abundance, [M/H]. The metallicity distribution function for red giant branch (RGB) stars in Leo I is confirmed to be very narrow, with mean value [M/H]  -1.2 and dispersion . By evaluating all contributions to the measurement error, we provide a constraint to the intrinsic metallicity dispersion, . We find a few metal-poor stars (whose metallicity values depend on the adopted extrapolation of the existing calibrations), but in no case are stars more metal-poor than [Fe/H] = -2.6. Our measurements provide a hint of a shallow metallicity gradient of -0.27 dex Kpc^-1 among Leo I red giants. The gradient disappears if our data are combined with previous spectroscopic datasets in the literature, so that any firm conclusions about its presence must await new data, particularly in the outer regions. By combining the metallicities of the target stars with their photometric data, we provide age estimates and an age-metallicity relation for a subset of red giant stars in Leo I. Our age estimates indicate a rapid initial enrichment, a slowly rising metal abundance – consistent with the narrowness of the metallicity distribution – and an increase of  dex in the last few Gyr. The estimated ages also suggest a radial age gradient in the RGB stellar populations, which agrees with the conclusions of a parallel study of asymptotic giant branch stars in Leo I from near-infrared photometry. Together, these studies provide the first evidence of stellar population gradients in Leo I.